Hopes for a breakthrough in the search for the missing Malaysia Airlines plane rose briefly Monday night when Vietnam scrambled helicopters to investigate a floating yellow object it was thought could have been a life raft.

But the country's Civil Aviation Authority later said on its website that the object turned out to be a "moss-covered cap of a cable reel", dashing hopes of authorities.

Distraught relatives of one of the passengers wait for news. Photo: AP

With the three-day search failing to find any confirmed trace of the plane or the 239 people aboard, fears are growing that the Boeing 777 may never be found.

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Authorities in Kuala Lumpur have ordered the search for the aircraft be intensified after they discounted that objects reportedly seen floating in the sea on Sunday were from the aircraft.

About 60 hours after the plane abruptly disappeared from radar, the search effort involving 46 ships and 34 planes from nine countries hit a dead-end.

Oil spills on the surface of the water off the southern seas of Vietnam possibly related to missing Malaysia Airlines flight MH370. Photo: AFP

One of the few hopes authorities have is that samples of an oil slick taken in the South China Sea on Sunday will be shown in chemical tests underway in Kuala Lumpur to be from the aircraft.

Oil spills from ships and exploration are frequent in the area.

Azaharuddin Abdul Rahman, head of Malaysia's Civil Aviation Department, told reporters the plane's disappearance is "puzzling" and perplexing" and the circumstances surrounding the flight's disappearance were "unprecedented."

A Vietnamese officer (right) and a reporter looking out the window during search operations over the southern seas off Vietnam. Photo: AFP

"We are intensifying efforts to locate the aircraft...we need positive evidence," he said.

Mr Azaharuddin stressed that authorities will not give up on the search, referring to an Air France jet that disappeared in the Atlantic in 2009. Its wreckage and crucial black box recorder were recovered two years after the crash.

Malaysia Airlines plane goes missing

A Malaysia Airlines Boeing 777-200 carrying 239 people lost contact with air traffic control early Saturday morning on a flight from Kuala Lumpur to Beijing, and international aviation authorities still hadn't located the jetliner several hours later. Photo: AFP

Mr Azaharuddin said the possibility of the plane having been hijacked had not been discounted, along with a number of other possibilities, including that it turned back two hours into the six-hour flight from Kuala Lumpur to Beijing.

A pilot flying another plane who tried to contact the pilots in the cockpit of the Malaysia Airlines plane said he heard mumbled voices before contact was lost.

The pilots made no distress call.

Is this the door from the missing Malaysian Airlines plane?

Mr Azaharuddin confirmed that since the plane disappeared from the radar screen no signal has been detected from its sophisticated equipment.

The Boeing 777 disappeared with 239 people, including six Australians and two New Zealanders, on board.

The plane lost contact with ground controllers between Malaysia and Vietnam after leaving Kuala Lumpur for Beijing.

A flight tracker screenshot shows the point where MH370 went off radar. Photo: AFP

Investigators suspect the aircraft might have disintegrated midair, partly because of the inability to find a concentrated pattern of debris.

But investigators have not ruled out any possibility, including terrorism.

The search area was expanded on Sunday after Malaysian defence officers reviewed radar logs indicating the plane may have turned around in flight, which would indicate it was experiencing some difficulty.

But the pilots did not send a distress call.

Failed to board

Four passengers on flight 370 failed to board after checking in their luggage, which raised further suspicion about the passengers after the plane disappeared.

But Malaysia’s Department of Civil Aviation chief Azaharuddin Abdul Rahman told Fairfax Media the passengers' luggage was offloaded from the plane before it left Kuala Lumpur airport in the early hours of Saturday morning.

He said the luggage was screened and found not to contain anything suspicious and was then returned to the passengers in the terminal.

"We followed standard operating procedures to remove the baggage of those who didn’t turn up," he said.

"There was nothing suspicious [about those passengers]," he said.

The identities of the four passengers have not been made public.

'Likely to have disintegrated'

"The fact that we are unable to find any debris so far appears to indicate that the aircraft is likely to have disintegrated at around 35,000 feet," said a senior source, who is involved in the preliminary investigations in Malaysia.

If the plane had plunged intact from such a height, breaking up only on impact with the water, search teams would have expected to find a fairly concentrated pattern of debris, said the source, speaking on condition of anonymity because he was not authorised to speak publicly on the investigation.

Finding traces of an aircraft that disappears over sea can take days or longer, even with a sustained search effort. Depending on the circumstances of the crash, wreckage can be scattered over many square kilometres. If the plane enters the water before breaking up, there can be relatively little debris.

The missing plane apparently fell from the sky at cruising altitude in fine weather, and the pilots were either unable or had no time to send a distress signal - unusual circumstances under which a modern jetliner operated by a professional airline would crash.

Malaysia's air force chief, Rodzali Daud, said radar indicated that the plane may have turned back, but did not give further details on which direction it went or how far it might have veered off course.

"We are trying to make sense of this," Mr Daud said at a news conference. "The military radar indicated that the aircraft may have made a turn back, and in some parts this was corroborated by civilian radar."

Malaysia Airlines chief executive Ahmad Jauhari Yahya said pilots were supposed to inform the airline and traffic control authorities if the plane made a U-turn.

"From what we have, there was no such distress signal or distress call per se, so we are equally puzzled," he said.

Stolen passports

Authorities were checking on the identities of the two passengers who boarded the plane with stolen passports. On Saturday, the foreign ministries in Italy and Austria said the names of two citizens listed on the flight's manifest matched the names on two passports reported stolen in Thailand.

"I can confirm that we have the visuals of these two people on CCTV," acting Malaysian Transport Minister Hishammuddin Hussein said at a news conference late on Sunday, adding that the footage was being examined.

"We have intelligence agencies, both local and international, on board."

Mr Hishammuddin declined to give further details, saying it may jeopardise the investigation.

"Our focus now is to find the aircraft," he said, adding that finding the plane would make it easier for authorities to investigate any possible foul play.

Interpol confirmed that at least two stolen passports used by passengers on the plane were registered in its databases. It said no one had checked the databases, but added that most airlines and countries did not usually check for stolen passports.

Mr Hishammuddin said only two passengers had used stolen passports, and that earlier reports that the identities of two others were under investigation were not true.

White House Deputy National Security Adviser Tony Blinken said the US was looking into the stolen passports issue, but that investigators had reached no conclusions.

In addition to the plane's sudden disappearance, which experts say is consistent with a possible onboard explosion, the stolen passports have strengthened concerns about terrorism as a possible cause. Al-qaeda militants have used similar tactics to try to disguise their identities.

Still, other possible causes would seem just as likely at this stage, including a catastrophic failure of the plane's engines, extreme turbulence, or pilot error or even suicide. Establishing what happened with any certainty will need data from flight recorders and a detailed examination of any debris, something that will take months if not years.

European authorities on Saturday confirmed the names and nationalities of the two stolen passports: one was an Italian-issued document bearing the name Luigi Maraldi, the other Austrian under the name Christian Kozel. Police in Thailand said Mr Maraldi's passport was stolen on the island of Phuket last July.

A telephone operator on a China-based KLM hotline on Sunday confirmed that "Maraldi" and "Kozel" were both booked to leave Beijing on a KLM flight to Amsterdam on March 8. Mr Maraldi was then to fly to Copenhagen, Denmark, on KLM on March 8, and Mr Kozel to Frankfurt, Germany, on March 8.

She said that, since the pair booked the tickets through China Southern Airlines, she had no information on where they bought them.

Having onward reservations to Europe from Beijing would have meant the pair, as holders of EU passports, would not have needed visas for China.

A team of American experts was en route to Asia to be ready to assist in the investigation into the crash. The team includes accident investigators from the National Transportation Safety Board, as well as technical experts from the Federal Aviation Administration and Boeing, the safety board said in a statement.

8 Mar
A Malaysia Airlines Boeing 777-200 carrying 239 people lost contact with air traffic control early Saturday morning on a flight from Kuala Lumpur to Beijing, and international aviation authorities still hadn't located the jetliner several hours later.

10 Mar
Two passengers on the missing Malaysia Airlines Flight MH370 were travelling on stolen passports and bought one-way tickets as authorities, including the FBI, investigate whether it was a terrorist act.

9 Mar
Investigations into the stolen passports of two passengers listed on missing Malaysia Airlines flight 370 have swung to the Thai resort island of Phuket, where there is a thriving stolen passport racket.

10 Mar
When a flight attendant utters the familiar phrase, ''in the unlikely event of an emergency'' in a pre-take-off airline safety demonstration passengers can be more certain than ever that it is not a platitude. And while it will provide cold comfort to the loved ones of the passengers aboard Malaysia Airlines Flight MH370, the reality is that such events are increasingly and remarkably rare.

10 Mar
Crime agencies say the discovery that two of the passengers aboard missing Malaysia Airlines flight 307 were using stolen passports has revealed a key flaw in international airline travel security.